A prior art scrambling apparatus is roughly classified into two types. In one type, the re-arrangement (scrambling) is carried out on a timebase, while in the other type, the re-arrangement is carried out on a frequency axis. In the scrambling apparatus for carrying out the rearrangement on the frequency axis, the signal series is divided into, for example, a plurality of frequency slots and the different frequency conversion is carried out at every slot. Upon re-arranging (descrambling), a frequency conversion reverse to the above frequency conversion is carried out to obtain the signal of the original frequency slot and thus the original signal series is obtained.
In the scrambling apparatus for performing the re-arrangement on the timebase, the signal series is divided into, for example, a plurality of frames. Then, these frames are re-arranged or a plurality of sampling data within the frame is re-arranged within a range of the frame. In the first example where the scrambling is performed on the timebase, the arranging and the rearranging on the timebase are performed as shown in FIG. 1.
In FIG. 1, reference numeral 1 designates a scrambling signal input terminal to which a signal to be scrambled (a scrambling signal) is inputted. The scrambling signal applied to this input terminal 1 is supplied through a trap circuit 2 and a low pass filter 3 to a PCM (pulse code modulation) encoder 4. The scrambling signal is pulse code-modulated by this PCM encoder 4, digitized and then fed to a scrambling circuit 5 in which the arranging on the timebase is performed. The data series thus scrambled is supplied to a composing circuit 6 in which it is added with a synchronizing signal which is supplied through a synchronizing signal input terminal 7. The synchronizing signal has the frequency of, for example, 2 KHz and is coincident with the characteristic of the afore-mentioned trap circuit 2. Namely, the scrambled signal is previously suppressed at its portion corresponding to the synchronizing signal. The data series passed through the composing circuit 6 is converted to an analog signal by a PCM decoder 8 and then supplied through a low pass filter 9 to a transmission path. This transmission path may be of either a wireless type construction or a wire type construction.
The low pass filters 3 and 9 are used to eliminate a noise and low pass filters 10 and 11, which will be mentioned later, are also used similarly. In the receiving system, the signal transmitted through the transmission path is delivered through the low pass filter 10 to a trap circuit 12 and a band pass filter 13. From this band pass filter 13 is derived the synchronizing signal of 2 KHz and this synchronizing signal is supplied to an automatic volume control circuit 14. The control signal therefrom is supplied to a PCM encoder 15 and in addition, the synchronizing signal itself is supplied to the PCM encoder 15. On the other hand, the signal series passed through the trap circuit 12 is PCM-modulated by the PCM encoder 15, which then is re-arranged on the timebase by a descrambling circuit 16. The data series thus re-arranged is converted to an analog signal by a PCM decoder 17 and delivered through the low pass filter 11 from an output terminal 18 to the outside.
By the way, in the above scrambling system, including the example shown in FIG. 1, the signal series to be transmitted can not just hide its appearance of the scrambled signal thoroughly regardless of the strength and weakness of the scrambling property, thereby urging a receiving person to decipher the code of the scrambled signal. As a result, the scrambling system which employs the simple code can be deciphered without difficulty. Accordingly, it becomes a trend to employ the scrambling system which requires an apparatus of high technology and thus the manufacturing cost thereof is inevitably increased. By way of example, the signal series is fourier-transformed at every frame by using an FFT (fast fourier transformer) and the frequency spectrum thereof is changed. After that, an IFFT (inverse fast fourier transformer) is employed to obtain the data series on the timebase which then is transmitted. In the receiving side, this data series is re-arranged by using the same apparatus. Of course, as long as the receiving person is going to decipher the code, any scrambling apparatus inevitably becomes useless after all.
In the second, the use of the scrambled signal sometimes becomes rude to the third person. When in, for example, the reception work and the like an information is exchanged in the presence of a visitor by employing the scrambled signal, if the scrambled signal is picked up by the visitor, the visitor inevitably realizes the scrambling property of the scrambled information when such information is exchanged.
These disadvantages become remarkable particularly when the scrambling system is applied to civilian goods such as an interphone and the like.